The invention relates to an assembly which is formed by a magnetic resonance apparatus, provided with a magnet system for producing a uniform magnetic field in a measuring volume, and a combination of coils for imaging an object to be examined which is to be arranged in the measuring volume, wherein
an orthogonal system of co-ordinates (x, y, z) is chosen in such a manner that its z direction extends parallel to the direction of the magnetic field,
the combination of coils includes a first coil and a second coil for the detection of a radio frequency signal emitted by the object, said coils having a first and a second region of sensitivity, respectively,
the first and the second coil include a respective longitudinal conductor, which longitudinal conductors extend parallel to a first plane (x-z) defined by the system of co-ordinates whereas their projections on a second plane (y-z) which is orthogonal to the first plane extend substantially parallel to the z direction,
the first and the second coil are oriented in space in such a manner that their longitudinal conductors are situated substantially one in the prolongation of the other and that the first region of sensitivity and the second region of sensitivity partly overlap.
An assembly of this kind is known from U.S. Pat. No. 5,664,568. The known magnetic resonance apparatus constitutes a device for the imaging of an object present in an imaging volume which is further defined as measuring volume. The magnetic resonance apparatus includes a magnet system which is necessary to produce a uniform main magnetic field B0 in a longitudinal direction (z direction) in the measuring volume. The magnetic resonance apparatus is provided with a detection means which are situated in the vicinity of the measuring volume in order to detect a radio frequency signal emitted by the object. In some cases use is made of two specific detection coils which are introduced into the magnetic resonance apparatus. For example, a birdcage coil is used for the imaging of the head whereas a surface coil can be used to image the spinal column. In some cases, for example for the acquisition images of the head together with the cervical vertibrae, however, it is desirable to arrange the two types of coils one in the prolongation of the other, so that the two imaging volumes partly overlap. The birdcage coil is provided with two electrically conductive loops which are interconnected by means of longitudinal electrical conductors. The signal-to-noise ratio of this coil is substantially constant within the volume of the coil and decreases strongly as soon as the edge zones are reached. The surface coil is shaped as a flat mat, the electrical conductors being arranged so as to achieve optimum sensitivity for the detection of the radio frequency signals emanating from the surface of the object. The electrical conductors of the surface coil are formed as turns, the individual signal-to-noise ratios in the turns being constant and equal whereas the signal-to-noise ratio decreases in the edge zones of the surface coil.
To those skilled in the art it will be generally known that when two coils are arranged in the vicinity of one another, a sensitivity disruption will occur in the transitional zone between the coils, thus leading to an unacceptably poor quality of the MR image of the relevant zone. This phenomenon occurs notably in the direct vicinity of the end portions of the longitudinal conductors of the coils.
The cited United States patent does not deal with the problem imposed by the sensitivity disruption so that it does not offer a solution to this problem either.
It is an object of the invention to reduce the sensitivity disruption in the transitional zone between two coils. This object is achieved by means of the assembly according to the invention which is characterized in that the first coil and the second coil have a first and a second signal-to-noise ratio, respectively, the first signal-to-noise ratio being higher than the second signal-to-noise ratio, and that at least one end portion of the longitudinal conductor of the first coil which faces the second coil is situated outside the y-z plane and extends in the direction away from the measuring volume. This solution is not coil-specific and can be used for all coil combinations exhibiting a difference in respect of signal-to-noise ratio.
It has been found that when the end portion of the longitudinal conductor of the coil having the highest signal-to-noise ratio is bent away from the y-z plane, the sensitivity of the relevant coil is varied so that the magnitude of the sensitivity disruption in the transitional zone can be influenced. It is to be understood that the necessary variation of the sensitivity of the first coil can be realized in various ways, that is, by selecting different ways of orienting the entire longitudinal conductor, or an end portion thereof, relative to the y-z plane. It has been found that this simple intervention significantly enhances the quality of the MR image. This solution does not require additional peripheral equipment and can be readily implemented.
It has been found that the sensitivity disruption can also occur around the end portions of the coils in the transitional zone between a coil and an environment. In conformity with the foregoing reasoning, the sensitivity disruption can be reduced by bending away the coils in the vicinity of the transitional area. This is achieved by means of an embodiment of the assembly according to the invention which is characterized in that the end portion of the longitudinal conductor of the first coil which is remote from the second coil is also bent away from the y-z plane. This step can also be taken for the second coil.
It has also been found in practice that the tuning of the coil must be constant as a function of time in order to sustain the required variation of the signal-to-noise ratio in the transitional zone. This is achieved by means of the assembly according to the invention wherein an end portion of the first coil and/or the second coil is arranged at least partly in a preformed, form-retaining fixture.